Question: $ E = \left[\begin{array}{rr}-1 & 0 \\ -1 & 4\end{array}\right]$ $ D = \left[\begin{array}{rrr}-2 & 0 & 1 \\ 2 & -2 & 1\end{array}\right]$ What is $ E D$ ?
Solution: Because $ E$ has dimensions $(2\times2)$ and $ D$ has dimensions $(2\times3)$ , the answer matrix will have dimensions $(2\times3)$ $ E D = \left[\begin{array}{rr}{-1} & {0} \\ {-1} & {4}\end{array}\right] \left[\begin{array}{rrr}{-2} & \color{#DF0030}{0} & \color{#9D38BD}{1} \\ {2} & \color{#DF0030}{-2} & \color{#9D38BD}{1}\end{array}\right] = \left[\begin{array}{rrr}? & ? & ? \\ ? & ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ E$ , with the corresponding elements in column $j$ of the second matrix, $ D$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ E$ with the first element in ${\text{column }1}$ of $ D$ , then multiply the second element in ${\text{row }1}$ of $ E$ with the second element in ${\text{column }1}$ of $ D$ , and so on. Add the products together. $ \left[\begin{array}{rrr}{-1}\cdot{-2}+{0}\cdot{2} & ? & ? \\ ? & ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ E$ with the corresponding elements in ${\text{column }1}$ of $ D$ and add the products together. $ \left[\begin{array}{rrr}{-1}\cdot{-2}+{0}\cdot{2} & ? & ? \\ {-1}\cdot{-2}+{4}\cdot{2} & ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ E$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ D$ and add the products together. $ \left[\begin{array}{rrr}{-1}\cdot{-2}+{0}\cdot{2} & {-1}\cdot\color{#DF0030}{0}+{0}\cdot\color{#DF0030}{-2} & ? \\ {-1}\cdot{-2}+{4}\cdot{2} & ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rrr}{-1}\cdot{-2}+{0}\cdot{2} & {-1}\cdot\color{#DF0030}{0}+{0}\cdot\color{#DF0030}{-2} & {-1}\cdot\color{#9D38BD}{1}+{0}\cdot\color{#9D38BD}{1} \\ {-1}\cdot{-2}+{4}\cdot{2} & {-1}\cdot\color{#DF0030}{0}+{4}\cdot\color{#DF0030}{-2} & {-1}\cdot\color{#9D38BD}{1}+{4}\cdot\color{#9D38BD}{1}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rrr}2 & 0 & -1 \\ 10 & -8 & 3\end{array}\right] $